Expert: The salt is likely to melt before the fuel rods get hot enough to crack

But this is uncharted territory, he says

(CNN) -- While the buildup of salt from seawater pumped in to cool reactors at Japan's Fukushima Daiichi nuclear plant may become a concern, it is likely to affect the pumping system itself before it affects the fuel rods, one expert said Thursday.

"In the core, it's probably not that great of a concern because it's going to be pretty hot in there," and the salt is likely to melt before the core reaches a dangerous temperature, said Gary Was, a nuclear engineering expert at the University of Michigan.

Seawater has been used as an emergency measure after the tsunami that followed the March 11 earthquake left the plant crippled.

When salt water boils and evaporates, it leaves the salt behind. There are some concerns that those salt crystals could adhere to the fuel rods -- insulating them against future efforts to cool them, allowing the temperature to rise and possibly resulting in overheating.

"For almost two weeks now, we've been injecting about 100 gallons a minute of seawater into these reactors," said Michael Friedlander, a former senior plant operator at three U.S. nuclear power plants who has been following the situation at the Fukushima Daiichi plant. He spoke on CNN's "AC360" Wednesday night.

The salt left behind "is now plating on surfaces inside the reactor vessel, as well as settling in the bottom of the reactor," he said.

Both Friedlander and Was pointed out that it's too soon to tell, given a lack of detailed information and instrumentation at the reactor, whether the salt crystals are a problem.

But, Was pointed out, salt itself liquefies at 800 degrees Celsius (1,472 degrees Fahrenheit). That is below the temperature at which zirconium fuel rods would oxidize and crack open, exposing the fuel. Oxidation becomes a self-sustaining process at about 900 degrees Celsius (1,652 degrees Fahrenheit). So, he said, it is likely the salt would melt and liquefy before that point is reached.

Where buildup could be a problem, he said, is in the system pumping in the cooler seawater, where lower temperatures are the norm.

Still, "this is uncharted territory," Was said, "so nobody really knows what the situation is in there, but we can make sort of educated guesses."

The picture will become clearer after full power is restored to the plant.

"We don't have any detailed information," Friedlander said. "We will get our first glimpses of that as they restore power to the main control rooms, as they get their instrumentation and controls back. From the main control rooms, we'll be able to monitor the levels and the temperatures in the spent fuel pool. So that's why restoring power was such a critical evolution and getting command and control back is going to be really important."

The seawater is being pumped into the cores of reactors No. 1 through 4 at the Fukushima plant in an effort to cool them and to replenish the spent fuel pools in at least two reactors.

Normally, seawater is about 3.5% salt, Was said.

In both the reactors and the spent fuel pools, the salt left behind as the seawater boils off is potentially concentrating the mixture, Friedlander said.